The invention relates to an improved process for the preparation of double metal cyanide (DMC) catalysts for the preparation of polyether polyols by polyaddition of alkylene oxides on to starter compounds containing active hydrogen atoms.
Double metal cyanide (DMC) catalysts for the polyaddition of alkylene oxides on to starter compounds containing active hydrogen atoms have been known for a long time (see, for example, U.S. Pat. No. 3,404,109, U.S. Pat. No. 3,829,505, U.S. Pat. No. 3,941,849 and U.S. Pat. No. A 5,158,922). The use of these DMC catalysts for the preparation of polyether polyols has the effect, in particular, of a reduction in the content of monofunctional polyethers with terminal double bonds, so-called mono-ols, compared with the conventional preparation of polyether polyols by means of alkali metal catalysts, such as alkali metal hydroxides. The polyether polyols obtained in this way can be processed to high-quality polyurethanes (e.g. elastomers, foams and coatings).
DMC catalysts are usually obtained by reacting an aqueous solution of a metal salt with the aqueous solution of a metal cyanide salt in the presence of an organic complexing ligands, e.g. an ether. In a typical catalyst preparation, for example, aqueous solutions of zinc chloride (in excess) and potassium hexacyanocobaltate are mixed and dimethoxyethane (glyme) is than added to the dispersion formed. After filtration and washing of the catalyst with aqueous glyme solution, an active catalyst of the general formula
Zn3[Co(CN)6]2.x ZnCl2.y H2O.z glyme
is obtained (see e.g. EP-A 700 949).
According to the prior art, DMC catalysts are prepared e.g. by mixing aqueous solutions of a metal salt (preferably of a zinc salt, such as e.g. zinc chloride) and a metal cyanide salt (e.g. potassium hexacyanocobaltate) in the presence of an organic complexing ligand (preferably tert-butanol) and optionally further ligands in a stirred tank to form a dispersion. The catalyst is isolated from the dispersion by known techniques, preferably by centrifugation or filtration. To achieve a sufficiently high catalyst activity it is necessary subsequently to wash the catalyst with an aqueous ligand solution. Water-soluble by-products, such as e.g. potassium chloride, which can reduce the activity of the catalyst, are removed from the catalyst by this washing step. According to the prior art this washing step is carried out by redispersing the catalyst in an aqueous ligand solution, e.g. in a stirred tank, with subsequent renewed isolation of the solid by e.g. centrifugation or filtration. To obtain highly active DMC catalysts it is in general necessary to wash the catalyst at least once more, non-aqueous ligand solutions preferably being used for the further washing operations. According to the prior art the further washing steps are also carried out by redispersing with subsequent isolation of the catalyst. Finally, the DMC catalyst must be dried. This form of catalyst preparation is exceptionally time-consuming and cost-intensive. Process times of more than 100 hours are required for preparation of DMC catalysts on a commercial scale (see e.g. U.S. Pat. No. 5,900,384). Because of the high catalyst costs, the profitability of the DMC-catalysed process of polyether polyol preparation is therefore considerably impaired.
It has now been found that highly active DMC catalysts can be obtained by a considerably simplified process in which aqueous solutions of a metal salt and a metal cyanide salt are first reacted in the presence of an organic complexing ligand a) and optionally one or more further complex-forming components b) to form a DMC catalyst dispersion, this dispersion is then filtered, the filter cake is subsequently washed by a filter cake washing and the washed filter cake is finally dried, after optional pressing out or mechanical removal of moisture.
This improved process for the preparation of catalysts avoids the several re-dispersions of the catalyst, with subsequent isolation, required according to the prior art to date for the preparation of highly active DMC catalysts and therefore leads to a considerable shortening of the process times for preparation of DMC catalysts. DMC catalysts which are prepared by the new, improved process have a comparable activity to DMC catalysts which are prepared in a significantly more expensive manner according to the prior art to date.